1. Field of the invention
This invention relates to a process for preparing industrially an .alpha., .beta.-unsaturated carboxylic acid or an ester thereof by using at least one ester selected from an .alpha.-hydroxycarboxylic acid ester, an .alpha.-alkoxycarboxylic acid ester and 8-alkoxycarboxylic acid ester as a starting material.
The .alpha.,.beta.-unsaturated carboxylic acid ester and the .alpha.,.beta.-unsaturated carboxylic acid are widely available in industry as starting materials for producing synthetic resins and reactive monomers, respectively. In particular, methyl methacrylate and methacrylic acid obtained from methyl .alpha.-hydroxyisobutyrate and/or methyl .alpha.- or .beta.-methoxyisobutyrate have been industrially important because methyl methacrylate can be used as a starting material for polymethyl methacrylate having excellent weather resistance and transparency and because methacrylic acid can be used as a starting material for various methacrylates.
2. Description of the Related Arts
As for a process of preparing .alpha., .beta.-unsaturated carboxylic acid and/or an ester thereof, a process in which a dehydrating reaction of an .alpha.-hydroxycarboxylic acid ester is carried out in a liquid phase, has heretofore been described in, for example, U.S. Pat. No. 3,487,101.
In addition, in Japanese Patent Application Laid-Open No. 184047/1985, a process is disclosed in which methyl methacrylate is prepared by reacting 90 to 100% concentrated sulfuric acid and methyl .alpha.-hydroxyisobutyrate in a liquid phase.
However, in the process for preparing methacrylic acid esters using sulfuric acid, there are great difficulties in their practical application on an industrial scale, because an extremely excess amount of high concentration sulfuric acid is required and the problem of processing a large amount of waste sulfuric acid diluted with water formed by the reaction is involved.
On the other hand, a process has been proposed for preparing methyl methacrylate from methyl .alpha.-hydroxyisobutyrate by a vapor phase catalytic reaction using a solid catalyst such as phosphate.
For example, in Japanese Patent Publications No. 0611/1969, No. 20612/1969 and No. 15724/1970, there are disclosed processes in which high boiling point materials, such as methyl .alpha.-hydroxyisobutyrate, methyl .alpha.-methoxyisobutyrate and methyl .beta.-methoxyisobutyrate, among the impurities contained in crude methyl methacrylate synthesized by the acetone cyanohydrin method, are passed through a catalytic layer, wherein solid phosphoric acid, alkali metal salts of phosphoric acid, or alkaline earth metal salts of phosphoric acid deposited on silica or silica-alumina to obtain methyl methacrylate and methacrylic acid.
However, when these phosphate-based catalysts are used, a high reaction temperature is required, so that deposition of a large amount of carbonaceous material and a side reaction, such as a hydrogenation reaction occur, causing problems in that, for example, methyl isobutyrate is formed as a by-product in large quantities, and therefore the above described processes are not satisfactory for practical use.